In typical electromagnetic clutches used for automotive type air conditioning compressors, a rotor or pulley is driven by a belt that encircles a drive pulley connected to the engine crankshaft. An electromagnetic clutch is usually mounted within the rotor, and the interior of the rotor acts as a core for a clutch coil assembly. A clutch armature plate is resiliently connected for axial shifting movement on the compressor driveshaft and has a friction surface in close proximity to a clutch end plate fixed to the rotor.
When the coil is energized, the clutch engages, and the armature plate is brought into frictional driving engagement with respect to the rotor so that rotor torque can be distributed to the compressor driveshaft. When the coil is de-energized, the clutch disengages, and the armature plate, along with the compressor driveshaft to which it is connected, are deactivated and remain stationary while the rotor continues to be continuously driven by the engine crankshaft whether the clutch is engaged or disengaged.
Since the rotor is continuously driven, if something happens in the air conditioning compressor that prevents the compressor driveshaft from rotating while the clutch is engaged, the clutch faces will slip past one another at high speeds. This causes a tremendous amount of heat to be generated. The heat, in turn, can cause serious damage to the clutch, the compressor and the drive belt. It is desirable, then, to prevent the generation of heat sufficient to cause serious damage until the overall unit can be serviced.
A mechanism that attempts to address this concern is disclosed in U.S. Pat. No. 4,896,756 to Matsushita. In the '756 patent, a compressor clutch has an armature plate made entirely of a low curie point material. A low curie point material is a magnetic temperature compensation material that has generally low magnetic reluctance, but becomes highly reluctant at temperatures above the material's curie point. The object of using this material is that when the material becomes highly reluctant above the curie point, this reduces the attraction force between the armature plate and a first rotatable member, thus reducing the clutch strength at the high temperatures. In this way, when a condition of slip between the clutch faces causes the temperature to rise above a certain temperature, the low attraction force reduces heat generation, thus avoiding extremely high heat conditions sufficient to damage the compressor and/or clutch.
The '756 patent further discloses meltable elements that are mounted within recesses of the annular armature plate and have protrusions disposed between the armature plate and a hub that determine the maximum amount of air gap between a first rotatable member and an armature plate. The meltable elements are designed to begin melting at about the curie temperature. So when the temperature is raised due to frictional heat from slippage, the meltable elements melt and when the clutch disengages, allow the air gap to increase to the point that the clutch cannot be re-engaged.
While this design attempts to solve the heat problem associated with clutch slippage, there are several drawbacks to this design. The entire armature plate is made of low curie point material. This is not necessary in order for low curie point material to be effective in reducing magnetic attraction. Rather than making the entire armature plate with low curie point material, the clutch armature plate or rotor need only be made with small segments of the special low curie point material strategically placed within the clutch magnetic flux path. This can be as small as one percent of the magnetic flux path being made of the low curie point material. If strategically placed, the small amount is as effective as a much larger portion of the clutch being made of low curie point material. The clutch flux path is the path that the magnetic field follows when the coil is energized. The rest of the rotor and armature plate can be made with conventional low cost low carbon steel, which has good wear characteristics.
The flux density along the flux path is greatest at certain points. Thus, the slippage protection can be accomplished with only one small segment of low curie point material strategically placed to increase the reluctance along the flux path at these points rather than making the entire clutch plate out of the special material.
Further, if the entire armature plate is made with low curie point material, then friction wear is a concern. Possible wear out of material is a higher risk since low curie point material is generally a much smoother material than conventional low carbon steel, which reduces good wear characteristics. Also, by strategically placing the low curie point material adjacent to the friction plate contact surfaces, but not contacting as a friction surface, the risk of exceeding the curie temperature during normal cycling of a healthy compressor is reduced.
Additionally, the '756 patent uses the low curie point material only on the armature plate. Yet, the pulley tends to heat more rapidly than the armature plate during clutch slippage. Therefore, there will be a better response to slippage in some instances if a portion of the pulley is made of the low curie point material.